Organic light-emitting diodes are light-emitting elements that use organic electroluminescence (hereafter, “electroluminescence” is abbreviated as “EL”), and generally have a structure in which conductive layers (an anode conductive layer and a cathode conductive layer) are provided on opposing surfaces of an EL layer containing a light-emitting layer which contains an organic light-emitting material. Besides the light-emitting layer, the EL layer may be provided with an electron transport layer or a hole transport layer or the like as required. Organic light-emitting diodes include bottom emission structures, in which an anode conductive layer formed from a transparent conductive material such as ITO, an EL layer, and a cathode conductive layer are formed sequentially on top of a transparent substrate such as a glass substrate, and the light is extracted from the substrate side of the structure, and top emission structures in which a cathode conductive layer, an EL layer and an anode conductive layer are formed sequentially on top of a substrate, and light is extracted from the opposite side of the structure from the substrate.
Organic light-emitting diodes offer a variety of advantages, including minimal view angle dependency, minimal power consumption and an ability to form extremely thin devices, but suffer from a problem of low light extraction efficiency. The light extraction efficiency is the ratio of the amount of light emitted into the atmosphere from the light extraction surface (for example, the substrate surface in the case of a bottom emission structure), relative to the amount of light emitted from the organic light-emitting material. For example, because the light from the light-emitting layer is emitted in all directions, the majority of the light enters a guided mode in which total reflection occurs repeatedly at the interfaces between a plurality of layers having different refractive indices, and this light is either converted to heat during guiding through the layers or is emitted from a side surface, resulting in a reduction in the light extraction efficiency. Further, because the distance from the metal cathode conductive layer is small, a portion of the near-field light from the organic light-emitting material is converted to a surface plasmon at the surface of the cathode and lost, resulting in a reduction in the light extraction efficiency. The light extraction efficiency effects the brightness of a display or illumination device equipped with the organic light-emitting diode, and therefore a multitude of methods for improving the light extraction efficiency are under investigation.
One method that has been proposed for improving the light extraction efficiency is a method that utilizes surface plasmon resonance. For example, Patent Documents 1 to 4 disclose methods in which a one-dimensional or two-dimensional periodic microstructure is provided on the surface of the metal layer (cathode conductive layer). The periodic microstructure formed on the metal layer surface functions as a diffraction grating, converting the surface plasmon into light at the cathode surface. As a result, the energy lost as surface plasmon can be extracted as light, thereby increasing the light extraction efficiency.
Of the above documents, Patent Document 4 discloses a method in which a substrate having a periodic lattice structure formed by surface unevenness is prepared using a dry etching method which employs a two-dimensional crystal body formed from a particle single layer film as an etching mask, and an anode conductive layer, a light-emitting layer and a cathode conductive layer are then stacked sequentially on top of the substrate. In this method, during the stacking process, the periodic lattice structure on the substrate surface is transferred sequentially to the upper layers, and therefore a periodic lattice structure having a shape that represents a reproduction of the periodic lattice structure of the substrate surface is formed on the surface of the cathode conductive layer on the side of the light-emitting layer.